Here is the full list of all 21 Skeletal Avenger achievements.
Collect 100,000 coins.
Find all item types.
Started New Game Plus.
Kill Sorcerer's Castle boss.
Kill Dark Caverns boss.
Kill 50 enemies with deflected projectiles.
Kill Sewer boss.
Complete a quest without using your weapon.
Complete a quest using only dash.
Find all trinkets.
Collect 10 perks for a single avenger.
Kill Dungeon boss.
Complete the tutorial.
Kill 8 enemies with a single primary attack hit.
Kill 8 enemies with a single skull throw.
Complete fifty quests.
Complete a quest without taking any damage.
Try out every kind of perk.
Have 100 enemies killed by environment hazards.
Complete a revenge in 150 seconds.
Kill 25 enemies within 10 seconds.